Sorry, that level of modification is beyond what we can help you with. If your main point is to be able to set the voltage more precisely, you can try replacing the 100k pot with a multi-turn pot that you can set more carefully. You should keep the wires as short as possible, make sure they don't short to anything, and even then, the performance of the regulator could suffer.
Regarding cascading regulators, it's probably going to be more efficient to just use one, but depending on the efficiencies of your two units at different operating points, cascading them could be more efficient. You could just try it both ways and see which is better in your application.